code: def test_source_add_start_token(self):
      reader = Seq2SeqDatasetReader(source_add_start_token=False)
      instances = reader.read(
          unicode(
              AllenNlpTestCase.FIXTURES_ROOT
              / "data"
              / "seq2seq_copy.tsv"
          )
      )
      instances = ensure_list(instances)
  
      assert len(instances) == 3
      fields = instances[0].fields
      assert [t.text for t in fields["source_tokens"].tokens] == [
          "this",
          "is",
          "a",
          "sentence",
          "@end@",
      ]
      assert [t.text for t in fields["target_tokens"].tokens] == [
          "@start@",
          "this",
          "is",
          "a",
          "sentence",
          "@end@",
      ]
  
file_path: magnitude/pymagnitude/third_party/allennlp/tests/data/dataset_readers/seq2seq_test.py
imports: from __future__ import division
  from __future__ import absolute_import
  import pytest
  from allennlp.data.dataset_readers import Seq2SeqDatasetReader
  from allennlp.common.util import ensure_list
  from allennlp.common.testing import AllenNlpTestCase
label_desc: Use image node_id 2 for calling the TestSeq2SeqDatasetReader obj's underlying member method code with example usage: obj.test_source_add_start_token() without return types